3. GALAXY FORMATION WITH HOT DM

The standard hot DM candidate is massive neutrinos
[18,
19,
20,
21],
although other, more exotic theoretical
possibilities have been suggested, such as a ``majoron'' of nonzero
mass which is lighter than the lightest neutrino species, and into
which all neutrinos decay. Neutrinos appeared to be an attractive DM
candidate because of the measurement of an electron neutrino mass of
about 30 eV in 1980
[22].
This coincided with the improving
CMB limits on the primordial fluctuation amplitude, which forced
Zel'dovich and other theorists to abandon the idea that all the dark
matter could be made of ordinary baryonic matter. The version of HDM
that they worked out in detail, with adiabatic Gaussian primordial
fluctuations, became the prototype for the subsequent
m = 1 CDM
theory.